The unique physico-chemical properties of carbon nanotubes (CNTs) has initiated extensive research into their possible applications in various areas of engineering (Baughman et al. Science 2002 297, 787-792; Rossi et al. Nano Letters 2004 4, 989-993; Tolles, W. M. and Rath, B. B. Current Science 2003 85, 1746-1759; Gogotsi, Y. Materials Research Innovations 2003 7, 192-194; Lin et al. Journal of Materials Chemistry 2004 14, 527-541).
The unique conductive properties of CNTs have attracted engineers to investigate methods to make CNTs magnetic as well (Gao et al. Carbon 2004 42:47-52; Coey, J. M. D. and Sanvito, S. Physics World 2004). To the best of the inventors' knowledge, previous attempts have had limited success.
For example, nanotubes with magnetic particles imbedded into CNTs during synthesis do not have useful magnetic properties. Further, the amount and location of the magnetic material inside the tube is difficult to control.
Techniques disclosed thus far for producing magnetic needles, are expensive, time-consuming (Singh et al. Journal of Nanoscience and Nanotechnology 2003 3:165-170), and have a relatively low yield. When increasing magnetization of magnetic needles, it is also important to prevent nanoneedles from agglomeration when magnetic field is not applied. Thus, encapsulating paramagnetic particles into CNT is desirable as it makes magnetic needles stable.
The phenomenon of spontaneous penetration of fluids into wettable capillaries has been taken as a guiding idea to load nanotubes with magnetic nanoparticles. As known from everyday experience, when a capillary is set in contact with a wetting fluid, the fluid spontaneously penetrates inside. This method of filling of nanotubes with molten metals has been suggested (Ajayan, P. M. and Iijima, S. Nature 1993 361:333-334; Dujardin et al. Science 1994 265:1850-1852; Ugarte et al. Science 1996 Science 274:1897-1899). Drawbacks of using the melts in this manner, however, include time consumption in that such methods are quite tedious and include the unpredictable step of nanotube opening. Further, the filling efficiency is too low to consider this method for large scale production. In addition, ferrous metals, which are of interest for magnetic applications, have high melting points (1565° C.) and can react with carbon.
U.S. Pat. No. 5,457,343 discloses nanometer sized carbon tubules enclosing a foreign material. The foreign material is introduced into the nanotube by forming an opening at the top portion of the carbon tubule either by contacting the foreign material with the top portion of the carbon tubule together with a heat treatment or by evaporation of the foreign material on the top portion of the carbon tubule together with heat treatment.
U.S. Pat. No. 5,543,378 discloses a carbon nanostructure having a palladium crystallite encapsulated therein.
U.S. Pat. No. 6,090,363 discloses a method for making carbon nanotubes open on at least one end by treating capped nanotubes with an oxidizing acid such as nitric acid. This patent also discloses methods for depositing materials into the carbon nanotubes once opened.
U.S. Pat. No. 6,129,901 discloses a method for producing uniform sized and uniformly aligned nanotubes and filling these nanotubes with metals using, for example electroless deposition.
Chemical vapor deposition (CVD) CNTs produced by template synthesis can be filled with water (Rossi et al. Nano Letters 2004 4:989-993; Naguib et al. Abstracts of Papers of the American Chemical Society Annual Meeting 2003 226:U535), ethylene glycol, hydrocarbons, and other liquids. Further, recent demonstration on a coulter-counter (Henriquez et al. Analyst 2004 129:478-482) shows that particulate flow inside nanotubes is possible.